To enable extraction of one specific optical fiber from a plurality of optical fibers at remote sites. An environment variable apparatus is mounted to provide a temperature difference at a predetermined position of an optical fiber. An environment variable apparatus is mounted to detect scattered light of light incident on the optical fiber through one end thereof so as to extract a target optical fiber from a large member of optical fibers. Further, synchronous devices are mounted to transmit from the environment variable apparatus to the extracting apparatus a message in that the temperature difference is given to the optical fiber, and transmit from the extracting apparatus to the environment variable apparatus a decision signal as to whether or not the optical fiber is to be extracted.

A fiber optic sensing coil formed in a polarization-maintaining optical fiber has two optical fiber leads extending therefrom. A multifunction integrated optics chip linearly polarizes optical signals input to the sensing coil. Fiber optic leads formed of polarization-maintaining optical fiber extend from the multifunction integrated optics chip. The fiber optic multifunction integrated optics chip leads are arranged such that the linear polarization of optical signals output from the multifunction integrated optics chip is directed along one of the principal axes of birefringence of each of the fiber optic multifunction integrated optics chip leads. Splices are formed between corresponding the first sensing coil leads and the multifunction integrated optics chip leads. The sensing coil leads and the multifunction integrated optics chip leads are arranged such that their corresponding principal axes of birefringence are at angles of approximately 45.degree. relative to one another. The first splice is formed a distance from the multifunction integrated optics chip such that a selected phase change between polarization components occurs in optical waves that propagate the distance L.sub.1 in the first multifunction integrated optics chip lead.

A spectrum stabilizer for stabilizing the wavelength of light emitted by a source for use in an optical system where the source can have that wavelength altered by varying one or more source parameters, such as source current, temperature and/or alignment of the output end of the source with an optical fiber of the optical system, and wherein light from a selected point in the optical system is coupled to a double mode waveguide for wavelength sensing.

The invention provides a method and apparatus for measuring one or more optical properties, such as absorbance and refractive index, of a test medium such as a gas, a liquid, or solid material. The method comprises providing a passive optical waveguide loop comprising the test medium, launching in the optical loop an intensity-modulated light at a reference phase, detecting a phase of said light along the optical waveguide loop, and comparing the detected phase of said light along the loop with the reference phase, wherein the comparison provides information about one or more optical properties of the test medium.

A piezoelectric fiber optic phase modulator having a mount at a nodal point of the vibratory system of the modulator. The modulator is excited with an electric field applied in a direction parallel to the axis of the modulator or perpendicular to the radial direction of movement of the modulator. In contrast to the mere hollow cylinder modulator of the related art, the modulator of this invention has a web or center structure integral with the inside surface of the cylinder. The modulator here has a rugged mount, low absorption vibration and negligible harmonics at the mount for efficient and high Q functioning, and has rugged electrical connections for dependable operation.